The ZX Spectrum+ 128Kexpansion connector is a double sided card edge connector with a 0.1 inch spacing. The two rows of conductors are numbered from right to left looking into the rear of the computer.
One pair of conductors are missing as there is an indexing slot cut out of the circuit board.

Contents

Notes on Connections

CPU Clock

The !CK signal, sometimes referred to as PHICPU is available on Lower Pin 8. This clock signal is generated by the ULA and is interrupted during contended memory access. This clock signal is inverted by a transistor switch to provide a clean clock edge for the Z80.

This signal is NOT connected on the Investronica Spectrum+ 128K.

Missing !IORQULA

Some schematics label Lower Pin 13 as !IORQULA, however this pin is NOT connected on either the Investronica or Sinclair Spectrum+ 128K.

Key Slot

The key slot ensures correct alignment of a peripheral with the edge connector. This slot is the width of one conductor and lies between Pin 4 and Pin 6, i.e. Pin 5 does not exist.

Power

Upper Pin 20 is connected to the -5 volt DC output of the internal power supply circuitry.

Upper Pin 22 is connected to the +12 volt DC output of the internal power supply circuitry.

Upper Pin 23 is connected to the oscillating voltage within the internal power supply circuitry. This is nominally 12 volts AC.

Lower Pins 6, 7, and 14 are connected to the 0 volt rail.

Removed Video Outputs

Unlike the ZX Spectrum 16k/48k the ZX Spectrum+ 128KULA generates RGB and Bright video signals rather than intensity and colour difference signals. The !Y, U, and V signals are therefore not available so the pins previously used for them are left unconnected. Additionally the composite video signal has been removed.